DIY Kallamazoo model 2 keeps blowing fuses at full AC

[goldstache]

Threw together a kallamazoo model 2 amp in an old chassis I had laying around.  I used original power transformer that was very close to the specs on the Kallamazoo.  I fired it up and it sounded ok.  Stock PT was getting pretty hot.  And there was a fair amount of audible noise in speaker as I brought the variac up slowly.  Kind of farty sounding.  The problem I thought would be the PT so I swapped it out for a new hammond 269JX.  I have built this amp before with this same transformer reconfiguring my HT supply/filters etc..  Again brought the amp up with the new PT and the same audible noise was there, even before filaments were active.  When I got to about 80% of my AC voltage on the variac.  The fuse blew!!!!!!   Rechecked my wiring.  Appears to be correct.  Replaced fuse and again at 80% AC supply it blew again. 


Any pointers on where to check current draw so see whats tripping the fuse.  Tried it without any valves in it and it stayed on at full AC supply.  Without any tubes in it was drawing a steady 12ma (AC) across the 1Amp fuse.  Put tubes back in same scenario.  Popped the fuse at about 80% AC supply and the PT is real hot.  Only had it on for about 30 seconds allowing time to bring the variac up incrementally. 


HELP!!!!!


Thanks

[PRR]

No tubes == OK

All tubes == pop

I wonder what happens with not-all tubes in?

[goldstache]

Pulled only EL84 and audible hum is still loud at 20% of AC supply (immediately).  PT is still getting hot quick and is gyrating a bit to the touch. 

[goldstache]

Just checked Data sheet on 269JX and it says regarding primary. 
White is 0V


Grey is 115V


Black is 125V


I wired off of the image on the XFMR box:


Top primary White
115V to Grey


Top Primary white
125V to black


I was under the impression that in the pictogram the top primary in the image was where hot goes and neutral to the black wire. 


I used alligators to try all different logical combinations and I always got a positive B+ voltage. 


Fuse still blowing.  Did I roast my PT?  any thoughts?

[goldstache]

I disconnected the PT secondaries and am getting nearly 600V with meter across them.  So I assume the transformer is operating correctly.  Still a lot of audible hum with just the heaters hooked up.  Must be gyration from the PT??????


Fuse did not blow under the conditions of this test.  Head scratching ensues.

[sluckey]

White wire from PT connects to white wire from power cord. Black wire from PT connects to black wire from power cord (probably thru a fuse and switch). Gray wire from PT connects to nothing. Tape it off.

[goldstache]

Thanks Sluckey.  I wired it that way, and with secondaries still unattached transformer was silent as i brought up the AC on the variac. 


Gator clipped the secondaries to the rectifier (SS) and brought the amp up.  Audible hum immediately from speaker, then at about 80% of AC supply.  POOF my last 1Amp fuse.  Guess Im done for the day. 


Should I unhook my heaters and just test transformer?

[goldstache]

Well I isolated the PT for testing.  All secondaries unattached.  B+ was ok.  Filaments, not so much.  Im getting 5.5VAC across the filament secondaries UNLOADED.  Did I fry them?  Im assuming an internal short would cause it to toast.


Bummer.  Wiring mishap on primaries causes that?  I've goofed before or a company changed their wiring color codes and that's never happened. 


Anybody got some info?


Thanks!

[HotBluePlates]

Well I isolated the PT for testing.  All secondaries unattached.  ...  Im getting 5.5VAC across the filament secondaries UNLOADED.  ...

At full primary voltage?

I'd doubt you could kill a good PT's filament winding in that manner with any error on the primary. It might be a new PT, but if you just bought it recently, I'd return it for a replacement.

[shooter]

secondaries to the rectifier (SS) and brought the amp up

did you start with SS?, I think the kazoo, was a tube rectified job

my last 1Amp fuse.  Guess Im done for the day.

you could OHM B+ to ground, should be many Kohms, and charging caps, might find something there

[terminalgs]

hot PT:  either the filament winding or the HT winding.  binary search time.. divide and conquer... If with tubes pulled and an SS rect, you still have too much amp draw on winding then (1)  Short (or near short) in the SS (incorrectly wired full-bridge rectifier) or (2) a bad OT, or (3) you have a resistor to ground from B+,  like one parallel to a filter cap or something that is too small a value (some people make it a habit to install bleeder resistors to bleed off B+ when the amp is off)  #3 seems unlikely unless you have a big wattage resistor, otherwise it'd be smoking...


since you hear noise in the speaker, #2 seems possible, test your OT...

[goldstache]

Thanks everyone!  My SS rectifier was on a terminal strip.  Smallest bit of wire from a shielded signal carrying wire, had flaked off, and was bridging 1 side of my 1N4007's to the ground lug of terminal strip.  Amp sounds great now.  I tried it with a 6X4 rectifier tube and SS.  Went switchable in the end. 
I was really pulling my hair out.  Only went over it 25 times.